The transmitter of a direct sequence spread spectrum (DS/SS) system spreads narrowband message signals into wideband signals using the spreading sequences. The receiver detects the desired signal by multiplying the received signal with the spreading sequences synchronized to that of the transmitter is disclosed in J. G. Proakis, Digital communications, McGraw-Hill, 3rd edition, 1995. To receive the signal, coherent receiver scheme needs the phase information of the received signal, providing better reception performance than noncoherent one at the expense of increased complexity. In particular, coherent detection in a rake receiver requires channel information including the amplitude and the phase of the channel. The channel information can be estimated using a pilot signal transmitted with the message signal. An example is disclosed in F. Ling, “Coherent detection with reference symbol based channel estimation for direct sequence CDMA uplink communications,” IEEE Proc. VTC '93, pp. 400–403, May 1993.
The accuracy of channel estimation can be improved by reducing the noise effect contained in the obtained channel information, which is usually achieved by employing a lowpass filter, called the channel estimation filter (CEF). The performance of channel estimation is significantly affected by the CEF and the channel condition, including the maximum Doppler frequency and fading characteristics. In general, commercial CDMA receivers employ conventional CEF with fixed parameters (e.g., the cut-off bandwidth).
A DS/SS receiver employing an adaptive channel estimator was proposed in Kitade et al., Spread spectrum receiving apparatus, U.S. Pat. No. 6,134,262, 2000, where the CEF is changed based on the maximum Doppler frequency. The maximum Doppler frequency was estimated by measuring how many times the received signal power exceeds a predetermined threshold. However, the scheme in said Kitade et al., Spread spectrum receiving apparatus, U.S. Pat. No. 6,134,262, 2000, may not be practical when the transmitted signal is fast power controlled for compensating fast fading as well as the near-far effect, because the power of the received signal cannot fully reflect the fading statistics of the channel.